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I'm responding to this very late, but in my experience, my Eheim filter has enough of a siphon going, I'm not sure if my bacteria would die with a power outage, obviously the flow rate goes down, so it wouldn't be able to filter the tank as well as normal, but the water keeps moving through at a decent clip.

I am responding later still, but...
I am trying to understand the comment about siphoning going on in response to a power outage. A siphon works because of gravity and requires no power to work, but your Eheim or any filter, needs power to complete the flow in a circular system. What am I missing? During a power outage the flow rate does not go down, it stops; there is none.

I am responding later still, but...
I am trying to understand the comment about siphoning going on in response to a power outage. A siphon works because of gravity and requires no power to work, but your Eheim or any filter, needs power to complete the flow in a circular system. What am I missing? During a power outage the flow rate does not go down, it stops; there is none.

Dacotah7, you didn't miss anything, your intuition was right. re reading what I posted there, it makes no sense whatsoever. I was thinking of the initial siphon I use to fill the filter in the first place, but obviously that only allows the filter to fill, not continue in some perpetual loop. I must have needed more sleep that day!

Great overall post on filtering systems and dead on but I think a word on bio-wheel HOB's needs to be added.

These are really hybrids - a HOB and a scaled down version of a wet/dry filter that fits on the back of the tank at water level. As noted, the oxygen level in the air is thousands of times higher than the water. The bio-wheel uses this function and also offers a versatile box (the HOB part) to put glass wool, and/or phosphate absorbers, extra bio-filtering media or charcoal for med removal that is easy to access all in one convent location - hence the versatility of this filter is par-one.

These filters cost a little more compared to run-of-the-mill HOB's and add a very slight amount of extra work (the spray bar needs to be cleaned now and again with the included “pipe” cleaner brush) but wil lcarry a heavier bio-load for the same volume tank due to the enhanced bio-level of action (due to the vastly improved oxygenation of the waste water.)

Combined with a canister on tanks 55 gal and greater, I can't think of a better combo.

Last edited by Cermet; 04-04-2011 at 01:01 PM.
Reason: typo

Knowledge is fun(damental)

A 75 gal with eight Discus, fake plants, and a lot of wood also with sand substrate. Clean up crew is down to just two Sterba's Corys. Filters: continuous new water flow; canister w/UV, in-tank algae scrubber!! Finally, junked the nitrate removal unit from hell.

There's an error in the description of the canister filters. While this error might be inconsequential to people who "just use" these filters, it might matter to those that want to properly understand their operation.

Canister filers are NOT gravity fed. It is a popular misconception. Again, it might matter to you or it might not.

If you want to see a good example of a true gravity feed, take a look at a typical wet-dry filter. The key moment here is that the water volume in a wet-dry filter is not continuous. There's a water pickup line and there's a return line. These two lines operate independently. They are separated by a gap: the filter itself, which is not sealed and is not flooded. The filter itself is essentially an air gap between the incoming and returning water volumes (which is why it is called wet-dry). And the pickup line of a typical wet-dry filter is indeed a good example of a gravity feed: the water is simply siphoned from tank under its own weight. This pickup process required no external power, which is the main feature of the gravity feed. Meanwhile, returning water from wet-dry filter back to the tank requires quite a bit of power (depending on the height difference and the volume per unit of time), so for high-performance wet-dry filters and large height differences you'd need a relatively strong pump. Note also, that if you disconnect a wet-dry filter from external power, it will overflow, since the gravity feed will continue to supply water, while the return pump is stopped.

Now, let's finally return to the topic of canister filters. The key detail about canister filters is that they are sealed and fully flooded units. In a canister filter, the water volume is continuous (!). The pickup line is filled with water, which leads directly into the fully-flooded inner volume of the canister and then directly to the return line. This water fully fills all available sealed volume (possible air bubbled notwithstanding). In this case the pickup and return lines are essentially two absolutely identical siphons, with absolutely identical gravity feed properties, acting on the same volume of water in opposite directions. These two siphons exactly and precisely counterbalance, cancel each other. Once the canister filter is set up (filled with water), the system cannot "gravity feed" itself (!). The system remains in exact equilibrium, with no self-sustaining feed of any kind possible. The beauty of this approach is that for the purposes of circulation the weight of the water in pickup line fully and exactly counterbalances the weight of the water in return line. Now, all you need to do to make the water circulate is to add a relatively simple water pump. From the filter pump's point of view, the water is weightless, regardless of how far below the aquarium level the filter is installed. This pump does not need to spend any effort to lift the water back to the tank, since the weight of the water is cancelled. All this pump has to be able to do is push the water through the media, i.e. overcome the fluid drag and friction in various hoses, chambers, cartridges, sponges etc. Again, and this is the key point: the pump does not need to spend any effort on lifting the water back into the tank, since in a canister filter the water is effectively weightless (for the purposes of circulation). If you disconnect a canister filter from external power, the water flow will simply stop. (Compare that to the above example with a wet-dry filter. See the difference?)

Note that because of this important property canister filters (as opposed to wet-dry filters) are fairly insensitive to the height difference between the tank and the filter itself. Regardless of how great that difference is, the siphoning action of the water in pickup line will exactly and precisely cancel the siphoning action of the water in return line. Longer hoses will, of course, create more fluid drag, but that's about it. That means that greater height difference does not require a more powerful pump (Compare it, again, to a wet-dry filter, where a greater height difference immediately requires more power from the pump.)

Some might ask: why then canister filter manufacturers always warn us about NOT creating too much of a height difference? There's a reason behind this, but it has absolutely noting to do with having to lift water to a greater height (as some people believe and, as you now know, it really not a factor at all). The real reason behind this is pressure. You see, regardless of how low you put your filter, the siphoning action of the water in the pickup line will always counterbalance the siphoning action of the water in return line (so the pump won't have to spend more effort). However, the pressure inside the filter will grow with the height difference. A canister filter is a sealed system and it is critically important to keep it fully sealed. All those sealing gaskets and rings inside the filter are rated to a certain level of pressure. Once you cross that limit, this will will start developing leaks. This is the reason you should stay withing the manufacturers recommended height difference. Again, crossing that limit will not kill your pump (as many people erroneously believe), but will rather kill your seals. (Basically, the reason for this is exactly the same some wristwatches have a depth rating, except that in a filter the water is inside)

Canister filers are NOT gravity fed. It is a popular misconception. Again, it might matter to you or it might not.

Of course, there's also such thing as so called "wet-dry canister" filters. When it comes to the principle of operation, such filers are essentially wet-dry filters masqueraded in a canister-like enclosure. These filters are, of course, gravity fed. What I said above applies, again, to ordinary fully flooded ("wet") canister filters.